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Created attachment 2624 [details]
Comparison what I thought vs. what I get

I have 'libsdl2-ttf-dev:amd64  2.0.14+dfsg1-1' which was not selectable in the bug report header.

The last argument "TTF_RenderText_Blended" takes is "SDL_Color fg". SDL_Color in turn has the members r,g,b,a. But member a is completely ignored with no mention in the documentation here https://www.libsdl.org/projects/docs/SDL_ttf/SDL_ttf_44.html

Either the alpha channel should be heeded or it should be explicitly mentioned that it isn't in the documentation, because the SDL_Color struct implies that fg.a will be processed. I hoped instead of the alpha channel of the rendering being in 0..255 that it would be in 0..fg.a, i.e. every alpha value would be multiplied by fg.a/255.

How I tested:

    /*
    g++ -g -std=c++11 -Wall -Wextra -Wshadow sdlttf-alpha-test.cpp $(sdl2-config --cflags --libs) -l SDL2_ttf && ./a.out
    */

    #include <SDL.h>
    #include <SDL_ttf.h>

    static bool useWorkaround = false;

    int main( void )
    {
        SDL_Init( SDL_INIT_VIDEO | SDL_INIT_TIMER );
        TTF_Init();

        int const screenWidth  = 400;
        int const screenHeight = 50;
        SDL_Window * pWindow = SDL_CreateWindow( "SDL2_TTF Alpha Test",
            SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, screenWidth, screenHeight, SDL_WINDOW_SHOWN );
        SDL_Renderer * pRenderer = SDL_CreateRenderer( pWindow, -1, SDL_RENDERER_ACCELERATED );

        SDL_SetRenderDrawColor( pRenderer, 40,40,40,255 );
        SDL_RenderClear( pRenderer );

        /* test TTF_RenderText_Blended alpha */
       TTF_Font * font = TTF_OpenFont( "Hack-Regular.ttf", 12 );
        for ( int nMax=32u, i = 0u; i <= nMax; ++i )
        {
            Uint8 const alpha = 255 * i / nMax;
            SDL_Color faceColor{ 255,0,0, useWorkaround ? Uint8(255) : alpha };
            SDL_Surface * const sString = TTF_RenderText_Blended( font, "+", faceColor );
            SDL_Texture * tString = SDL_CreateTextureFromSurface( pRenderer, sString );
            SDL_Rect dstRect{ screenWidth/2 - nMax*10/2 + i*10, screenHeight/2 - sString->h / 2, sString->w, sString->h };
            if ( useWorkaround )
                SDL_SetTextureAlphaMod( tString, alpha );
            SDL_RenderCopy( pRenderer, tString, NULL, &dstRect );
        }
        TTF_CloseFont( font );

        SDL_RenderPresent( pRenderer );
        SDL_Delay( 2000 );
        SDL_DestroyRenderer( pRenderer );
        SDL_DestroyWindow( pWindow );
        SDL_Quit();

        return 0;
    }


P.S.: In the linked documentation there is a typo: "sntialiased" instead of "antialiased".